Appendix D
The National Health and Nutrition Examination Survey

HISTORY

“The National Health Survey Act of 1956 authorized the secretary of the Department of Health, Education, and Welfare (now the Department of Health and Human Services), acting through the National Center for Health Statistics (NCHS), to collect statistics on a wide range of health issues. Given this directive, NCHS has conducted health examination surveys for more than 20 years. Among other topics, NCHS collects statistics on determinants of health and on the extent and nature of illness or disability of the U.S. population.

“In 1960–62, the first National Health Examination Survey was conducted. The sample population for this survey was adults 18–74 years old. Two additional surveys were conducted during the 1960s on children 6–11 years old and adolescents 12–17 years old, respectively.

“In 1971, the range of topics included in the survey was extended to include nutritional status. Indexes of nutritional status were to be obtained through a medical history, a dietary interview, a physician’s examination, medical procedures and biochemical tests, and body measurements. This survey, the first National Health and Nutrition Examination Survey (NHANES I), was conducted between 1971 and 1974. NHANES II, conducted between 1976 and 1980, extended the age range to include infants 60 months to 1 year old.

“NCHS conducted the Hispanic Health and Nutrition Examination Survey (NHANES, 1982–1984) of persons of Mexican-American, Puerto Rican, and Cuban ancestry residing in the southwestern U.S., the New York City area, and Dade County, Florida. The next national survey, NHANES III and, in the tradition of the past national surveys, continues to be a keystone in providing critical information on the health and nutritional status of the U.S. population.



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Monitoring Human Tissues for Toxic Substances Appendix D The National Health and Nutrition Examination Survey HISTORY “The National Health Survey Act of 1956 authorized the secretary of the Department of Health, Education, and Welfare (now the Department of Health and Human Services), acting through the National Center for Health Statistics (NCHS), to collect statistics on a wide range of health issues. Given this directive, NCHS has conducted health examination surveys for more than 20 years. Among other topics, NCHS collects statistics on determinants of health and on the extent and nature of illness or disability of the U.S. population. “In 1960–62, the first National Health Examination Survey was conducted. The sample population for this survey was adults 18–74 years old. Two additional surveys were conducted during the 1960s on children 6–11 years old and adolescents 12–17 years old, respectively. “In 1971, the range of topics included in the survey was extended to include nutritional status. Indexes of nutritional status were to be obtained through a medical history, a dietary interview, a physician’s examination, medical procedures and biochemical tests, and body measurements. This survey, the first National Health and Nutrition Examination Survey (NHANES I), was conducted between 1971 and 1974. NHANES II, conducted between 1976 and 1980, extended the age range to include infants 60 months to 1 year old. “NCHS conducted the Hispanic Health and Nutrition Examination Survey (NHANES, 1982–1984) of persons of Mexican-American, Puerto Rican, and Cuban ancestry residing in the southwestern U.S., the New York City area, and Dade County, Florida. The next national survey, NHANES III and, in the tradition of the past national surveys, continues to be a keystone in providing critical information on the health and nutritional status of the U.S. population.

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Monitoring Human Tissues for Toxic Substances This information is essential for estimating the prevalence of various diseases and conditions, elucidating mechanisms of disease development, and planning for health policy” (NRC, 1984b). PURPOSES “The fundamental purposes of the NHANES are “to develop information on the total prevalence of a disease condition or a physical state; to provide descriptive or normative information; and to provide information on the interrelationships of health and nutrition variables within the population groups” (R.S.Murphy, Director, Health Statistics Branch, National Center for Health Statistics, DHHS, personal communication). Additional purposes of the NHANES are “to measure the health and nutritional status of the U.S. population and specific subgroups and to monitor changes in health and nutritional status over time. “Thus, the surveys have provided estimates of the prevalence of characteristics or conditions in the American population, and normative or descriptive data have been developed, such as data on weight and stature. Through successive surveys, repeated collection of these estimates permits the assessment of changes in health and nutritional status over time” (NRC, 1984b). “The data obtained in NHANES surveys have been extremely important in providing needed information on the prevalence of various health conditions and the distribution of physical, psychological and biochemical characteristics in the U.S. population. However, since many health effects are slow-acting consequences of the long-term exposure to a combination of environmental, dietary, social and demographic factors, the importance of each of these interacting factors can only be known by following the same individuals longitudinally over a period of time. Thus, a followup study of the original adult population of NHANES I, the Epidemiologic Followup Survey, was begun in 1982. “The primary purpose of the NHANES I Epidemiologic Followup Survey was to investigate relationships between the presence of certain environmental, nutritional, social, psychological and demographic factors and the occurrence of specific diseases. Westat, under contract to the National Center for Health Statistics, attempted to locate and interview 14,407 adults who were age 25–74 at the time they were examined in the original NHANES I survey, which was conducted approximately 10 years earlier. The Followup Survey was funded by the National Institute on Aging (NIA) with additional support from other National Institutes of Health institutes and HHS agencies” (Chu and Waksberg, 1988).

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Monitoring Human Tissues for Toxic Substances DESIGN General Features “One of the purposes of the NHANES II program with respect to nutritional status assessment required that the program continue to use, with some modifications, the same format as NHANES I. To monitor nutritional status, the data collected needed not only to be comparable (at least largely), but also, as in NHANES I, to be collected on a probability sample of the noninstitutionalized civilian population of the United States. “The general structure of the NHANES II sample design was therefore similar to that of NHANES I. The design was that of stratified, multistage, probability cluster sample of households throughout the United States. The sample selection process involved a number of factors, including the selection of primary sampling units, household clusters, and households. A primary sampling unit (PSU) was a primary location, generally a county of small group of contiguous counties, from which sample housing units and sample persons were selected. When the definition and stratification procedures were completed, 64 PSUs throughout the United States were included in the NHANES II survey plan. “The clinical examinations and other procedures were conducted in specially designed mobile examination centers. These mobile centers were moved from location to location in a predetermined fashion to achieve economy of operation and to avoid the North in the winter. At any time during the survey period, two mobile examination centers were operating (in different locations) while a third was being relocated. These mobile centers provided a controlled, standardized environment for the clinical examinations and tests. Thus, the clinical procedures could be conducted by a trained staff that moved from site to site with the mobile centers. “Because of the small number of mobile centers, the logistic constraints involved in moving and setting up the centers, the large number of subjects, and the length of each examination, the total period for data collection was 4 years. The average length of an examination was 2–3 hours, but examinations varied, depending on the age of the subject. For example, the examination of a preschool child lasted no more than 2 hours, and that for an adult no more than 3 hours” (NRC, 1984b). Statistical Design “The survey was designed to produce statistics for four broad geographic

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Monitoring Human Tissues for Toxic Substances regions of the Untied States and for the total population by sex, age, race, and income classification. NHANES II was a probability sample of the civilian, noninstitutionalized population of the United States aged 6 months through 74 years. “For nutritional assessment, three population groups of presume higher risk of malnutrition were of special sampling interest. These groups were pre-school children (6 months to 5 years old), the aged (60–74 years old), and the poor (persons below the poverty levels defined by the U.S. Bureau of Census). These three groups were oversampled to improve the reliability of the statistics generated about them. Although women of child-bearing age were also considered to be at risk of malnutrition, oversampling of them was not necessary, because adequate numbers were included in the sample. A total of 21,000 examined persons was desired as the sample size. The number selected from each of the 64 PSUs was to be between 300 and 600.” “In an initial interview conducted in the household, sociodemographic information and medical histories were collected. A visit to a mobile examination center was then scheduled for each subject. At the mobile center, the physical examination, dietary interview, anthropometry, and other procedures and tests were conducted. “In the end, 27,801 persons were interviewed and 20,322 persons were examined in the 1976–1980 NHANES (NHANES II). Because not all interviewed persons were examined, appropriate statistical adjustments for nonresponse were made. These adjustments brought the sum of the final sample weights into close alignment with sex, age, and race estimates of the Bureau of the Census at the midpoint of NHANES II” (NRC, 1984b). METHODOLOGY IN NHANES I, II, AND HHANES “In all three cycles of HANES (i.e., NHANES I, NHANES II and HHANES) the samples of persons were selected through complex, multistage, highly stratified, cluster samples of households. The selection of sample persons involved the designation of primary sampling units (PSU’s), enumeration districts (ED’s), households, eligible persons, and finally sample persons (SP’s). Each PSU consisted of a county or a small contiguous group of counties; ED;s consisted of households in a small geographic area within a PSU. The target population for NHANES I was the civilian noninstitutionalized population under 74 years of age residing in the coterminous United States, excluding residents of reservation lands set aside for use of American Indians. Alaska and Hawaii were added to the target population of NHANES II. For HHANES the target population consisted of Mexican-Americans, Puerto

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Monitoring Human Tissues for Toxic Substances Ricans, and Cubans living in selected areas of the U.S. A summary of the designs in all three HANES is given in Table 1–1” (Chu and Waksberg, 1988). DATA COLLECTION METHODS “The five major components of NHANES II were a household questionnaire, a medical-history questionnaire, dietary questionnaires, a physical examination by a physician, and special clinical procedures and test, including x rays and test on samples of blood and urine. “The household questionnaire asked for information on family relationship; some demographic items, such as sex, age, and race of family members; housing; occupation, income and educational level of each family member; and participation in the food stamp program and the school breakfast and lunch program. Two medical-history questionnaires were used; one questionnaire was used for children 6 months to 11 year old, and a different one for persons 12–74 years old. Both household and medical-history interviews were conducted in the respondent’s home. “The dietary interview, physical examination, and special clinical procedures and test (depending on the age of the subject) were conducted when the subject arrived at the mobile examination center. The procedures and tests included body measurements (all subjects); skin-prick tests for allergy (persons 6–74 years old); x rays of cervical spine, lumbar spine(except for women under 50), and chest (persons 25–74 years old except for pregnant women); urine (persons 6–74 years old); and blood (all subjects)” (NRC, 1984b). QUALITY ASSURANCE “The quality of the dietary component of the survey was monitored at several levels. All interviews were conducted by dietary interviewers who had at least a bachelor’s degree in home economics. Before the survey began, the interviewers were trained both in interview techniques and in coding of food items identified in the 24-hour dietary recall. A manual issued to each interviewer described the procedures to be followed. “To promote consistency in quality control, dietary interviewers periodically reviewed and evaluated each other’s work. At every location, each interviewer tape-recorded two randomly selected subject interviews. These recordings were then evaluated for adherence to procedures. “Randomly selected 24-hour recall forms were manually reviewed at headquarters before programmatic edits were completed. To detect errors and

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Monitoring Human Tissues for Toxic Substances unusual results form a particular location or from a particular dietary interviewer, the NHANES headquarters staff reviewed the “ranges” of nutrient intake for each respondent” (NRC, 1984b). DATA PROCESSING “Preparation of data and reports of the NHANES II findings involved several steps. With some items, such as x-ray pictures, interpretations were required to produce data units that could then be coded. Coded data were key punched into machine-readable form. The data were edited and validated. Sampling weights—the designated number of people in the population represented by a survey subject—were determined. For selected measures, imputation procedures for item nonresponse were developed and reviewed. Data were then analyzed and reports were developed” (NRC, 1984b). DATA REPORTING “Descriptive, analytic, and methodologic reports were published in Vital and Health Statistics (Series 1,2, and 11), a publication of the NCHS. All completely edited, validated, and documented tapes were released for public use through the National Technical Information Service. Information has also been made available in journal articles and in presentations at professional meetings” (NRC, 1984b). RESEARCH INITIATIVES “NCHS research is of two kinds: statistical analyses describing the potential contribution of demographic, biologic, and other variables to determinants of health and nutritional status’ and development, testing, and transfer of methods involved in conducting surveys and validating data from them. This research is conducted intramurally (that is, within DHHS), often in conjunction with the Food and Drug Administration (FDA), Centers for Disease Control (CDC), or the National Institutes of Health (NIH). No extramural research, as with a grants program, is supported directly by NCHS. Although research in support of the surveys is needed, little funding is available” (NRC, 1984b).

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Monitoring Human Tissues for Toxic Substances NHANES III “In preparing for NHANES III, survey planners have solicited suggestions from federal agencies, the Congress, the public-health and nutrition communities, researchers, foundations, and associations. A variety of mechanisms have been considered to gather recommendations from these groups, including letters, meetings, and notices in journals” (Chu and Waksberg, 1988). MAJOR FACTORS AFFECTING THE DESIGN FOR NHANES III “A number of important factors had an impact on the development of the sample design for NHANES III. Among these were: (a) the existence of alternative sampling frames and whether they were suitable for NHANES III; (b) the definition of domains of study and corresponding precision requirements; (c) the desired level of effort” (Chu and Waksberg, 1988). PRECISION SPECIFICATIONS AND DOMAINS FOR ANALYSIS “There are three basic components of the sample size specifications for NHANES III: The first related to the minimum sample size for specified subdomains. This minimum was set at 560 examined SP’s for all subdomains. The 560 was established to ensure the desired reliability even for statistics that were subject to relatively high design effects (see table 1–2). Superimposed on the cell-size requirement was a provision that the total number of examined SP’s in the age-sex groups below were at least as large as the numbers show below. These sample sizes are necessary for the growthcurve analyses planned for NHANES III. The third involved the distribution of the total sample size by race-ethnicity. The required sizes (in terms of examined persons) are: Total 30,000 Black 9,000 Mexican-American 9,000 White and all other 12,000 The set of subdomains for which specified reliability was desired consisted of sex-age groups for the largest race/ethnicity populations in the U.S. The

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Monitoring Human Tissues for Toxic Substances age groups differed among the race/ethnicity domains, and are shown in table 1–3. The subdomains consist of the age groups shown, separately for males and females. There are, thus, 52 subdomains, twice the number of age groups” (Chu and Waksberg, 1988).